Slugging a five groove barrel- Is there an easy way to measure?

[Reggie Perrin]

So I just picked up a BSA "Martini Cadet". Like many of them in the US, the barrel was rechambered from the original .310 Greener to .32-20 when it was imported.

According to multiple sources on the internet, BSA Cadets are supposed to have a groove diameter of .316", and firing factory .32-20 in these guns is only to be done if you enjoy inaccuracy and bore leading.

I wanted to slug the bore to verify its dimensions before ordering a mold. Something I don't remember any of the stuff I read about these guns mentioning is the fact that this thing has five groove rifling.

As someone who does not own a machine shop with thousands of dollars worth of specialized measuring equipment, how does one go about measuring a slug from a five groove barrel? Is there a simple way to determine the actual groove diameter?

 

[Art Eatman]

If no help within three or four days (please wait), start a new thread in the Rifle forum, General.

 

[FrankenMauser]

Dardas Cast Bullets, as well as several other companies, will measure 5-groove slugs for free. Just send it to them with the contact information that they request.


The process is simple, and all you need is a V-block and a micrometer. ...But some people prefer to avoid the necessary math based on the Pythagorean Theorem.

 

[Reggie Perrin]

I didn't know that. Thanks for the link. I read about the V-block method, but what I read said you needed a 108* V for a proper measurement of a five groove. I suppose I could have a machine shop make one for me, but since I only plan on doing this once, just sending a slug to Dardas seems an easy and painless (and cheaper) solution.

 

[Jim Watson]

RCBS makes a heel bullet for the .310 that I have read will work in the .32-20 conversions.
http://rcbs.com/Products/Bullet-Casting/Bullet-Moulds/Rifle/Bullet-Mould-310-120-RN-N-R.aspx

There was a lot on the subject on castboolets and the CBA board about 4 years ago.

 

[SGW Gunsmith]

Find someone locally who has an "odd-flute micrometer", like any machine shops in your area. Bet they'll be willing to measure that slug and give you a result in 0.0001 of an inch accuracy.

 

[Unclenick]

Reggie,

The ideal tool is a 108° V-block micrometer, used to measure the diameter of five-flute end mills. As SGW says, most machine shops will have them. I got a used one on eBay some time back. A little chipped at the carbide edges, but usable. CDCOtools.com has a foreign-made one for $105, but I expect that's more than you want to invest.

You can often do it on a 90° block as the groove impressions in the slug are usually wide enough to let it work out, but it takes some math or a couple of pin gauges to get the height ratio worked out. I can describe it if you have a V-block and the slug fits it OK.